(1) Field of the Invention
The present invention relates to the manufacture of integrated circuits in general, and in particular, to a functional tool for determining the etch rates and time to etch as a function of chemical properties as well as the thickness of the material that is being etched to form integrated circuits.
(2) Description of the Related Art
It is usually the common practice that in order to remove a given amount of material by etching, the material is subjected to a corresponding time of etching. That is, for a set xe2x80x9cfilm loss targetxe2x80x9d, such as in terms of the film thickness, a xe2x80x9ctime to etchxe2x80x9d is calculated knowing the etch rate for that particular etching process. Then, the same calculated etch time is used from run-to-run. However, it will be appreciated by those skilled in the art that the etch rate, and hence the etch time will vary from run-to-run, even for the same material that is being etched with the same etch recipe, because conditions that affect etching, such as temperature, concentration, and other factors will vary from run-to-run. With the same etch time, therefore, films may be over etched, or, damaged. Hence, it is the purpose of the present invention to provide a method of calculating variable etch rates as a function of the conditions of the etching process, as well as the variable thickness of the material that is being etched, as disclosed later in the embodiments of the invention.
In prior art, various characterizations of wet etch are shown. In U.S. Pat. No. 6,004,881, Bozada, et al., disclose a wet chemical digital etching technique for gallium arsenide or other semiconductor materials. Hydrogen peroxide and an acid are used in a two step etching cycle to remove the gallium arsenide. In the first step of the cycle, gallium arsenide is oxidized by 30% hydrogen peroxide to form an oxide layer that is diffusion limited to a certain thickness. The second step of the cycle removes this oxide layer with an acid that does not attack unoxidized gallium arsenide. These steps are repeated in succession using new reactant materials and cleaning after each reactant until the desired etch depth is obtained. A method for using a Hall effect measurement to determine the achieved change in surface layer thickness of doped semiconductor material is also taught.
In another U.S. Pat. No. 5,582,746, Barbee, et al., teach a contactless method and apparatus for real-time in-situ monitoring of a chemical etching process during etching of at least one wafer in a wet chemical etchant bath. The method comprises the steps of providing two conductive electrodes in the wet chemical bath, wherein the two electrodes are proximate to but not in contact with a wafer; monitoring an electrical characteristic between the two electrodes as a function of time in the etchant bath of the at least one wafer, wherein a prescribed change in the electrical characteristic is indicative of a prescribed condition of the etching process; and recording a plurality of values of the electrical characteristic as a function of time during etching. From the plurality of recorded values and corresponding times, instantaneous etch rates, average etch rates, and etching end points are determined.
What is needed, however, is a method of determining etch end points where the conditions of the bath, such as the temperature and the concentration of the etching solution, as well as the properties of the film that is being etched are also taken into account. Such a method is disclosed later in the embodiments of the present invention.
It is therefore an object of the present invention to provide a method of determining wet etch points.
It is another object of the present invention to provide a method of determining etch rate as a function of bath temperature and solution concentration.
It is yet another object of the present invention to provide a method of determining etching time, or time to etch, as a function of bath temperature, solution concentration and the thickness of the film to be etched.
It is still another object of the present invention to provide a method of determining wet etch points for a target film loss or for a target film thickness.
These objects are accomplished by providing a rate formula ER(C, T)=ER0*C*exp(xe2x88x92Ea/RT)+A; providing a substrate; providing an etch bath in a tank; providing an etch concentration monitor in said tank; providing a temperature monitor in said tank; forming a film over said substrate; measuring the initial thickness of said film over said substrate; placing said film on said substrate in said tank; starting the etching of said film in said tanks, and simultaneously recording the start time of said etching; measuring a first temperature of said etch bath in said tank; measuring a first concentration of said etch bath in said tank; stopping said etching of said film, and recording the stop time of said etching; removing said substrate with said film thereon from said tank; measuring the final thickness of said film on said substrate; calculating a first film loss by subtracting said initial thickness from said final thickness of said film; calculating a first etch time by subtracting said start time from said stop time of etching; calculating a first etch rate (ER(T,C)1) as a function of said first temperature and first concentration; repeating previous steps at second, third, (nxe2x88x921) and n temperatures while holding constant said first concentration, and calculating the corresponding second, third, (nxe2x88x921) and nth etch rates [ER(T)C2, . . . ER(T)C(nxe2x88x921), ER(T)Cn] values as a function of said temperatures, and their statistical average (ER(T)C,avg.); repeating previous steps at second, third, (nxe2x88x921) and n concentration while holding constant said first temperature, and calculating the corresponding second, third, (nxe2x88x921) and nth etch rates [ER(C)T2, . . . ER(C)T(nxe2x88x921), ER(C)Tn] values as a function of said concentrations, and their statistical average (ER(C)T,avg.); using said values in said formula to calculate etch rate ER for subsequent etching of said film for a specified film loss or to a specified film thickness; providing a substrate having formed thereon a first film to be etched and a second film, wherein a first film thickness o said first film is less than a second film thickness of said second film; calculating a film loss xcex94H for said specified film loss or for said specified film thickness; and calculating etch time.
These objects are further accomplished by providing a rate formula ER(C,T)=ER0*C*exp(xe2x88x92Ea/RT)+A; rewriting said formula in a simplified form as ER(C)=K*C+A; determining the values of first constant K and second constant A in said formula for sand etching solution; using said values of said first and second constants in said simplified equation to calculate an etch rate; providing a substrate having formed thereon a first film A to be etched and a second film B, wherein a first film thickness of said first film A is HA, and a second film thickness of said second film B is HB; calculating a film loss xcex94H; and calculating etch time.